1. Technical Field of the Invention
The present invention relates in general to the synchronization of a receiver with a transmitter in a cellular radio communications system, and in particular to improving stand-by time of a mobile terminal by minimizing the time of reception during paging.
2. Description of Related Art
Code Division Multiple Access (CDMA) systems were developed to allow multiple users to share scarce radio communication resources. In CDMA systems, a number of users are allocated to the same frequency. To distinguish among signals for different users, each user is assigned a pseudo-noise spreading sequence, which a transmitter uses to spread a narrow-band information to a wide-band signal. To detect a CDMA signal, a conventional receiver despreads the received signal using the same spreading sequence that was used by the transmitter.
WCDMA is an improvement of the CDMA method and has been developed by various organizations around the world. Some of the benefits provided by WCDMA include support for increased bandwidth and bitrates, and provisions for improved packet data communications and other services.
An important consideration for mobile terminals in a communications system is that of stand-by time. Stand-by time of a mobile terminal represents the maximum battery life of a mobile terminal when not in use for a conversation. To increase the stand-by time of a mobile terminal, the power consumption when the mobile terminal is turned on but not in use should be minimized. Power consumption can be minimized by powering off different components in the mobile terminal, such as signal processing circuitry, when the mobile terminal is not in use. When the mobile terminal is idle, its main task is to monitor a paging data channel and respond to any pages received. The base station utilizes the paging channel to send out paging messages or notifications of incoming calls to the mobile terminals. To minimize the time the mobile terminal requires to demodulate the paging data channel, an additional channel denoted as the paging indicator channel is transmitted. The mobile terminal is required to monitor the paging indicator channel periodically to demodulate the paging indicator. During those periods when the mobile terminal is not required to monitor the paging indicator channel, the mobile terminal typically conserves power by powering off its receiver.
In WCDMA systems, synchronization of a mobile terminal with the base station is done using a common pilot channel, which transmits known pilot symbols in parallel to a paging indicator channel and a paging data channel. These various channels are separated by different orthogonal codes. To monitor the paging indicator channel, the mobile terminal must first synchronize its receiver with the base station through a process of carrier frequency correction and detection of the channel impulse response, or path profile. By powering up the receiver, de-spreading the received signal using a common pilot code, and using a path searcher, the mobile terminal can perform carrier frequency correction and identify the relevant radio paths. If the synchronization process including the path search is too short, the receiver will not be adequately synchronized with the base station, which can result in errors in detecting the paging indicator. On the other hand, by using a longer path search, a more accurate radio path estimate and thus better synchronization can be achieved. However, any excess time used during the path search implies that the mobile terminal has to be turned on for a longer period of time before the actual paging message is received, thereby reducing the stand-by time of the mobile terminal.
There is a need, therefore, for a method of minimizing the time of reception during paging when the mobile terminal is communicating in a paging mode while, at the same time, providing for sufficient synchronization to minimize errors in the detection of pages.